1. Field of the Invention
This invention is concerned with seismic sound source arrays such as arrays of air guns. In particular, it is concerned with the simulation, in real time, of the far field signature of the array, based upon the instantaneous field conditions that exist at the time the array is activated. Such art may be found in classes 367/20-23.
2. Discussion of the Prior Art
As is well known in the art of marine seismic surveying, a sound source is towed behind a ship beneath the surface of a body of water. The sound may be generated by a small explosive charge, an electric spark or arc, a vibrator or, preferably, an array of several air guns. The air guns each contain a volume of air compressed to about 2000 psi or more. Upon command, the guns abrupty release their volumes of compressed air to create a thunderous sound wave in the water. The resulting pressure wavefield propagates downwardly, into the earth beneath the sea floor, to the sub-bottom strata, whence the wavefield is reflected back up towards the water surface. The reflected wavefield is detected by a hydrophone array that it towed behind the ship just beneath the water surface. The hydrophone array may extend three thousand meters or more behind the ship and may include several thousand hydrophones. The detected reflected wavefields are recorded on time-scale recordings or seismograms.
When the seismic source it triggered or fired, it produces a complex output pressure pulse. The hyrophones feel that pressure pulse and convert the pressure variations to an electrical wave train. Typically, the electrical wave train consists of a short, initial, fast positive rise in amplitude, followed by several rapidly-decaying oscillations. The wavetrain might be 150 to 200 milliseconds (ms) long and it termed the "signature" of the sound source.
The wavefield generated by the sound source radiates by spherical spreading in all directions. There is a downwardly-travelling component as well as an up-going component. The air-water interface is an excellent reflecting surface with a reflection coefficient that may approach -1. The up-going component of the wavefield is reflected from the water surface, is reversed in polarity and becomes another downwardly-travelling wavefield, popularly known as the ghost.
The seismic sound source array is usually towed about 6 to 10 meters beneath the sea surface. Assuming a water velocity of 1500 meters per second (mps), the vertical two-way time lag between the direct primary wavefield and the ghost will be about 8 to 14 ms. The ghost interferes, in opposite polarity, with the direct wavefield to create a complex source signature. That circumstance is termed the ghost effect. The ghost is an integral part of the source signature.
The significance of the ghost effect is explained in considerable detail in U.S. Pat. No. 4,658,384, issued 04/14/87 to W. H. Dragoset et al., which is incorporated herein by reference. In that patent, it is shown that in the near field, that is, within a few tens of meters, the ghost distorts the source signature differently than in the far field. The far field is defined as that distance from the source at which the amplitude ratio between the ghost and the direct wavefield equals or exceeds 0.95. For all practical purposes, the far field is considered to exist at distances in excess of 250-300 meters from the source. As explained in the '384 patent, for various reasons, it is usually impractical to attempt to measure, experimentally, the far-field signature of a sound source array. The far-field signature is more conveniently extrapolated from near-field experiments.
One such extrapolation method is taught by the '384 patent. Another reference of interest is U.S. Pat. No. 4,648,080, issued 03/03/87 to N. D. Hargeaves. Other references of interest are U.S. Pat. Nos. 4,644,507 issued 02/17/87 to A. M. Ziolkowski, 4,476,550 issued 10/84 to Ziolkowski et al., 4,500,978 issued 02/19/85 to Ziolkowski et al., 4,326,271 issued 04/20/82 to Ziolkwski.
Instead of using standard source arrays such as would be employed in routine operations, the references required special source and receiver arrangements to make the near-field measurements. Furthermore, in the references, it was assumed that environmental and firing conditions such as sea state, source power output and fire-time delays remained ideal and/or constant. The alleged far-field signature derived by the prior art was applied willy-nilly in all data-processing algorithms as though it were Gospel but without regard to the changing operating conditions that are inevitably present during a routine geophysical survey.
As an example, consider the effect of wave height on an array measuring 30-50 meters on a side. In heavy seas with large swells, one air gun of the array might easily be five or six meters deeper than a gun at the other end of the array (see FIG. 2). The ghost would arrive at the deeper gun 8 ms later than the ghost arriving at the other gun. A composite of the individual source signatures of the array taken as a whole, would not resemble a source signature as derived from the previous-art methods that assumed ideal, constant conditions.
During the course of a seismic project, many different lines of survey are occupied. Each line may offer different operating problems that require different array configurations. Obviously, the far-field signature derived for one array configuration will not be the same as one that is derived for some other configuration. The far-field signatures must be individually determined for each line of survey to fit changing environments.
It is a purpose of the invention to provide a method for deriving the far field signature of an array of sound sources in real (or near real) time, without use of specialized equipment. The method takes into consideration the instantaneous firing and environmental conditions that exist for each of the individual units of the array at the time the array as a whole is triggered.